Receivers for communication systems generally are designed such that they are tuned to receive one of a multiplicity of signals having widely varying bandwidths and which may fall within a particular frequency range. It will be appreciated by those skilled in the art that these receivers intercept electromagnetic radiation within a desired frequency band and convert the intercepted electromagnetic radiation into an electrical signal. The electromagnetic radiation can be input to the receiver by several types of devices including an antenna, a wave guide, a coaxial cable, an optical fiber, and a transducer. Through appropriate filtering techniques the desired portion of the electrical signal is selected and subsequently processed by either analog or digital signal processing techniques.
These communication system receivers may be capable of receiving signals known as narrow band or wide band signals; however, such receivers generally utilize circuitry which is duplicated for each respective signal to be received which has a different bandwidth. Thus, in such a receiver, a narrow band signal would only pass through a narrow band filter and the wide band signal only would pass through a wide band filter. The output of each filter subsequently would be selected depending upon the mode of operation that is desired for the receiver. The disadvantage to this type of receiver is that circuitry must be duplicated for each desired signal having a different bandwidth which is to be received. In addition, the bandwidth of each desired signal must be known a priori so that the filtering and signal processing stages may be properly designed. If a receiver is designed for a specific channel bandwidth and a new communication service is later introduced which requires a different bandwidth (i.e., a different signal coding and channelization standard) which was not anticipated, then a completely new receiver capable of supporting this new bandwidth will need to be designed and utilized.
An alternative receiver structure is possible which would be capable of receiving either narrow band or wide band signals. This alternative receiver may utilize a digitizer which operates at a sufficiently high sampling rate to ensure that the wide band signal can be digitized in accordance with the Nyquist criteria (i.e., digitizing at a sampling rate equal to at least twice band width to be digitized). Subsequently, the digitized wide band signal preferrably is processed using digital signal processing techniques. The digitizer typically also includes an anti-aliasing filter which is sufficiently wide to pass the wide band signal. Thus, the receiver is essentially designed as one which uses a wide bandwidth detector or digitizer.
The post digitizer digital signal processing algorithms may process the single output from the digitizer such that the wide band signal is operated upon for whatever filtering or detection criteria is desired, such that a set of narrow band filters and associated algorithms to effect a multi-channel narrow bandwidth receiver may be applied to this single digitized output. Such a structure is essentially equivalent to a single wide band front end in the receiver followed by a bank of narrow bandwidth filters to provide a number of outputs. It will be appreciated by those skilled in the art that one possible technique for providing this type of receiver structure is the use of Discrete Fourier Transforms (DFT's) or similar digital filtering techniques, to synthesize a series of adjacent narrow bandwidth filters after digitization.
The disadvantage to this alternative type of receiver is that the digitizer portion of the receiver must have a sufficiently high sampling rate to ensure that the Nyquist criterion is met for the maximum bandwidth of the composite received channel which is equal to the sum of the individual adjacent narrow band channels comprising the composite bandwidth. If the wide bandwidth signal is sufficiently wide, the digitizer may be very costly and may consume a considerable amount of power. Furthermore, high performance receivers which require high dynamic range also typically require the digitizer to internally generate only low levels of spurious signals. This low spurious signal requirement is difficult to achieve with practical wide bandwidth digitizers, especially if the digitizers have a multiplicity of signals input with potentially large variations in the signal power levels of the individual signals which are to be received. Additionally, the channels produced by a DFT filtering technique must typically be adjacent to each other and as such the maximum bandwidth over which such a receiver may be operated and still process a multiplicity of narrower bandwidth channels is necessarily restricted to N times the number of possible narrow bandwidth channels.
Therefore, a need exists for a receiver capable of receiving a wide band signal within a channel and a multiplicity of narrow band signals within corresponding channels with the same receiver circuitry. This receiver circuitry preferrably should not include the use of a high rate digitizer having low level spurious noise constraints. This receiver circuitry also preferrably would allow independent tuning of the receiver to individual narrow band channels at a desired center frequency. In addition, the narrow band channel reception should be configured such that the receiver outputs may be used individually as narrow band channels or in combination with each other to synthesize a wide band channel.
Such a flexible receiver architecture would, for example, be ideally suited for cellular radio communication systems. Currently, cellular operators are developing plans to operate service regions within their cellular systems in accordance with one or more information signal coding and channelization standards (i.e., air interface standards). If the cellular operators were to use current receiver design techniques, then a new receiver would have to be designed and built for each new information signal coding and channelization standard. However, this approach may be inadequate or at the very least not cost effective for cellular operators during the present and likely future business climate in which cellular operators are pressured to adopt new standards more quickly to satisfy growing demand for more and better communication services. As a result of these pressures cellular operators may find it desirable to deploy a receiver in their service regions which can be reconfigured, at will, to simultaneously receive one or more different types of information signals which may be present as electromagnetic radiation within a particular frequency band with the same receiver circuitry, rather than separate receiver circuitry for each type of information signal. These information signals may be coded and channelized as frequency division multiple access signals, time division multiple access signals, frequency hopping code division multiple access signals, or direct sequence code division multiple access signals. Some of these coding and channelization approved and proposed standards have been given specific names including: Advanced Mobile Phone Service (AMPS), Narrow Advanced Mobile Phone Service (NAMPS), Total Access Communication System (TACS), Japanese Total Access Communication System (JTACS), United States Digital Cellular (USDC), Japan Digital Cellular (JDC), Groupe Special Mobile (GSM), Direct Sequence Spread Spectrum (DS-SS), Frequency Hopping Spread Spectrum (FH-SS), Cordless Telephone 2 (CT2), Cordless Telephone 2 Plus (CT2 Plus), and Cordless Telephone 3 (CT3).